1. Field of the Invention
This invention relates to the art of filtering particulate solids and other deleterious matter from liquids, and more specifically, to an improved method for improving the filtering capacity of a granular medium filter such as a sand filter. Very effective wastewater filters have been developed and described in applicant's prior U.S. Pat. Nos. 3,459,302; 3,516,930; 3,587,861; 3,792,773; 3,817,378; 3,840,117; 4,032,443; 4,627,923; 4,818,414 and Re. 28,458. These patents are incorporated by reference herein to the general art to which the present invention is directed.
2. Information Disclosure Statement
Granular medium filters such as sand filters are widely used to remove particulate material and other deleterious matter from wastewaters, potable water supplies, and the like. Such filters trap fine particulate matter within the interstices of the filtering medium, while particles larger in size than the interstices are separated as a layer on the filter medium surface. Eventually, the flow of water through the filter bed is hindered by these trapped and separated solids so that the liquid level above the bed rises. This increased resistance to the movement of liquid through the filter medium bed is a result of reduction in porosity of the bed medium.
The trapped and separated materials are removed periodically by stopping the flow of influent water and backwashing the filter bed with previously-filtered water. The filter normally is backwashed when resistance to flow through the filter bed results in the liquid above the filter bed rising to a predetermined level.
Extending the filtration time between backwashing has several advantages. First, the total quantity of solids loaded onto the filter bed between backwashes is increased. With a constant volume of backwash water, the solids concentration in the backwash water is increased. This reduces the overall hydraulic load placed on subsequent backwash water treatment equipment. Normally backwash water is recycled to the head end of a treatment plant, adding to the hydraulic load and solids load of the plant. Reducing the backwash frequency results in a reduced hydraulic load to the plant although the solids load from the filter backwash remains unchanged. The backwash water is more easily treated when it comprises a small volume containing concentrated solids. This is best accomplished by increasing the solids capture with resultant longer filter runs between backwashes. Second, downtime is minimized, thus maximizing effective filtration time.
Several methods have been used to prolong the filter run length between backwashes, without using additional filter area. Ross U.S. Pat. No. 3,459,302 discloses a granular medium filter in which currents are created in the liquid above the filter bed surface by an air diffuser. These currents sweep across the surface, removing solids trapped at the medium surface, and maintain them in suspension in the liquid above the filter surface. This technique to increase filter capacity and filter bed porosity is termed "air scour".
Another very effective means for reducing the frequency of backwashing is described in Ross U.S. Pat. No. 3,817,378. In this reference, at the time filter bed porosity decreases due to separated solids, and liquid accumulates above the filter bed, volumes of air are forced upward through the medium in intermittent pulses of short duration. Some variations of this procedure are known as "air pulse". A portion of the filtered solids is forced into liquid suspension above the filter bed, while another portion is concentrated by surface medium movement into localized sites within the bed itself. Thus, some of the solids are "stored" within the filter bed, reducing the quantity of solids which produce the flow resistance at the bed surface. This filter cleaning operation is generally repeated a number of times between backwashes, greatly extending the filtration time before backwashing is required.
The quantity of filtered solids which may be stored in the bed without adversely affecting the filtration rate is considerable, but limited. This generally restricts the numb of pulses which may be advantageously performed between backwashes.
A variation of the "air scour" and "air pulse" design is disclosed in Ross U.S. Pat. No. 4,627,923. A plurality of hydraulic jets within the filter bed employ filtered liquid to pulse the granular bed medium. Hydraulic jets above the filter bed also create currents in the liquid to maintain solids in suspension during the filter run.
Garzonetti, in U.S. Pat. No. 4,693,831, discloses a method for controlling the pulsing of a granular medium filter based on determination of the rise rate of liquid level above the filtration medium. Either air or liquid is used to pulse the filter bed.
In U.S. Pat. No. 4,859,330 Pauwels discloses a traveling bridge device with air scour and backwash means for successively cleaning each of a plurality of filter cells formed in a filtration tank.
As currently practiced, the "air scour" system or the "above bed hydraulic jet" system are activated when the liquid level above the filter bed rises to activate a first level detector. This detector is generally situated at a low position in the filter tank. Activation of either of these systems results in removing some of the solids from the filter surface and suspension of these solids in the liquid above the filter bed. As additional solids accumulate on and in the filter bed, the liquid level above the bed rises further to activate a second level detector. The second detector activates the "air pulse" or hydraulic jet pulse system. The "air pulse" or hydraulic jet pulse system results in volumes of air or liquid being forced upward through the medium in intermittent pulses of short duration. A portion of the filtered solids is forced into liquid suspension, while another portion is concentrated by surface medium movement into localized sites within the bed itself. Thus, some of the solids are "stored" within the filter bed, reducing the quantity of solids which produce the flow resistance at the bed surface. Additional pulses are initiated on a timed and timely basis at preset intervals of a few to many minutes, depending upon the expected solids loading and hydraulic loading.
Backwashing is typically initiated when a third level detector, located at a level higher than the first and second level detectors in the filter tank, is activated by the rising liquid level. Influent wastewater flow ceases and filtrate is forced up through the filter medium bed at a fluidizing or sub-fluidizing velocity. The backwash water carries the collected particles out of the filter cell, and generally back to the head of the treatment plant. The backwash flow ceases and the washed granular medium settles back to form a filter bed and another filtration cycle commences.
For the above described granular medium filters, the filter cell is a unit capable of individual operation to perform the described filtration process. Although not limited in dimensions, the cell generally ranges in size from 4 ft..times.5 ft. (20 ft.sup.2) to 30 ft..times.50 ft. (1500 ft.sup.2) and is equipped with pumps, valves, piping and controls to operate independently. Commonly, a number of filter cells operate in parallel to accept continuous flow from a treatment plant. One or more filter cells may be undergoing backwash or be closed for service while the remaining cells continue to filter the wastewater.
During the period of the air pulse or jet pulse in a filter cell, downward flow through the filter medium is temporarily retarded, and the liquid level over the medium bed will rise. The rate of rise is a function of the rate of inflow to the filter cell. For example, with an influent flow rate to a filter cell of approximately 7.5 gallons/min/ft.sup.2, the rate of level rise is approximately 12 inches per minute. A pulse period 30 seconds in length results in the liquid level rising 6 inches. This level rise may not be recovered during the period subsequent to the pulse period, should the influent rate be very high. The liquid level may be raised even further by subsequent pulse cycles, and prematurely activate the backwash cycle. To overcome this problem several modifications to the filter systems described above have been invented.